The present invention relates to a control mechanism for the pull-down pawl of a film projector, for the projection of films with different frame frequencies, in which a control drum with a plurality of cam curves is provided for controlling the movement of the pull-down pawl into and out of perforations of the film.
Known constructions of such control mechanism require a relatively large space near the objective, especially if different frame frequencies have to be obtained. In addition, the transmission of the control movement from the control drum to the pull-down pawl causes considerable difficulties. If, as usual in the known constructions, the plurality of cam curves on the control drum are tracked by a transmission member, movable in axial direction of the control drum to be aligned with a selected one of the cam curves, then this movement of the transmission member in axial direction will produce, depending on position of the transmission member, a different control movement of the pull-down pawl. In order to assure a secure engagement of the pawl into the perforations of the film, a larger stroke is necessary when the transmission member engages a cam curve adjacent the pawl, then when the transmission member engages a cam curve which is located closer to the tilt bearing of the pull-down pawl. This transmission relationship between the control movement at the respective cam to the movement of the end of the pull-down pawl means into and out from the film perforations will also change the angle of inclination of the sections of the respective cam curve which serve to control the movement of the end of the pull-down pawl means. With the change of the aforementioned transmission relationship, the influence of the tolerances, at which the cam curves are produced, changes likewise. All these factors evidently make the production of the control drum considerably complicated, while the control mechanisms known in the art do not operate in a fully satisfactory manner.